Enable droidstubs to depend on aconfig_declarations_group module type

This change enabled aconfig_declarations_group module type to be added
as `aconfig_declarations` property of droidstubs module. Similar to
aconfig_declrations module, the aconfig_declarations_group module
provides the intermediate cache files of the aconfig_declarations
modules of the listed codegen modules to droidstubs module.

Test: m nothing --no-skip-soong-tests
Change-Id: I42eb64d3ab65e679ea733c5bb98833971e49a8e8